Glasses with Tunable Liquid Crystalline Order

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NON-TECHNICAL SUMMARY Liquid crystals (LCs) and glasses are important materials in modern technologies. LCs enable rapid adjustment of molecular organization by external control, making them useful for displays and sensors. Glasses merge liquid-like disorder and spatial uniformity with the mechanical strength of a solid, finding applications in optics, in electronics, and as amorphous pharmaceuticals. This project, supported by the Solid State and Materials Chemistry program as well as the Condensed Matter Physics program at NSF, creates new materials that combine the advantages of both LCs and glasses - glassy materials with continuously variable LC order. These materials display molecular packing that is tunable between the order of a LC and the disorder of a glass, while maintaining the advantages of ordinary glasses. They are expected to find applications in organic electronics and optoelectronics. For example, the ability to control the organization of emitter molecules in the active layers of organic light emitting diodes (OLEDs), which are widely used in cell phone displays, can significantly enhance the efficiency of such displays. Given the number of these devices this is a benefit of global scale. In general, these new materials are ideal for applications where performance depends on molecular orientation and packing and but can be compromised by defects commonly present in crystalline materials. Additionally, pharmaceutical scientists will be able to employ these materials to optimize solubility and physical stability. Furthermore, the project advances the training of graduate, undergraduate, and high school students by integrating materials research and education. Outreach activities include the offering of a materials chemistry course each summer to high school seniors from under-represented groups through the UW-Madison PEOPLE program, a short course titled Amorphous Pharmaceuticals to industrial scientists each May through UW Extension Services, and new demonstrations on crystalline and glassy materials for Science Is Fun, an outreach program led by ASU scientists and delivered throughout the Phoenix Metro Area.TECHNICAL SUMMARYLiquid crystals are known for their fast and reversible phase transitions. The speed at which these transitions occur has led to the notion that fast cooling will not suppress LC ordering, but recent work has discovered that LC transitions can be dynamically arrested at practical cooling rates, thus allowing the formation of glasses with variable LC order or the access to different LC phases. This project, supported by the Solid State and Materials Chemistry program as well as the Condensed Matter Physics program at NSF, establishes the principles by which molecular organization can be controlled through conditions of glass preparation. The team tests the hypothesis that the amount of LC order trapped in a glass is determined by the kinetic arrest of the end-over-end rotation of rod-like LC molecules. LC order is evaluated as a function of temperature and cooling rate by X-ray scattering conducted using both laboratory and synchrotron sources. Dielectric spectroscopy determines the timescales of molecular rotations about the different axes and the temperatures of kinetic arrest. Aging experiments are used to study the stability of LC order in the glassy state. This project is expected to provide a general recipe for selecting process conditions to control LC order in a glass, leading to materials that have the potential to enhance the performance of organic electronics and pharmaceutical formulations. Current theories of LC transitions generally do not include time as a variable, effectively assuming instantaneous transitions controlled by thermodynamics alone. Results from this project will support development of theories in which transformation kinetics play an important role.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.